Foam, such as polystyrene foam, is commonly made by combining a physical blowing agent with molten polystyrene resin under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
From about the 1950's to the present, physical blowing agents of choice have been halocarbons, hydrocarbons, or combinations thereof. Examples of these include commercially available halocarbon compositions, such as dichlorodifluoromethane and trichlorofluoromethane, and the C.sub.4 -C.sub.6 hydrocarbons. These physical blowing agents, however, have various disadvantages. For example, physical blowing agents are released into the atmosphere during and after foam production and, thus, are a source of pollution. In addition, the hydrocarbon blowing agents constitute a fire hazard.
In the past, carbon dioxide has been experimented with as a blowing agent for resins, such as polystyrene resins. Carbon dioxide has been listed as a blowing agent or as a co-blowing agent in the following: U.S. Pat. No. 3,431,164 to Gilbert et al., U.S. Pat. No. 4,198,363 to Noel, and U.S. Pat. No. 4,470,938 to Johnson. Carbon dioxide has a disadvantage of a very high foaming rate caused by its high volatility. A very high foaming rate results in a high degree of corrugation in the extruded sheet. Because of the high degree of corrugation, a lower amount of blowing agent is used as compared to using a conventional hydrocarbon or halocarbon blowing agent. When a lower amount of blowing agent is used, higher viscosities and a lower output rate are obtained.
Carbon dioxide has been used as a co-blowing agent or a diluent for hydrocarbon or halocarbon blowing agents. This is illustrated, for example, in U.S. Pat. No. 4,344,710 to Johnson et al., and in an article by L. M. Zwolinski and F. J. Dwyer, "Extruded Polystyrene Foam With CFC/Carbon Dioxide Blowing Agents," ANTEC '86, pages 30-33, Conference Proceedings, Society of Plastic Engineers 44th Annual Technical Conference and Exhibit. Because of the high volatility of carbon dioxide, it can only be substituted at a low level in the process. Thus, the use of carbon dioxide as a co-blowing agent with hydrocarbon or halocarbon blowing agents still results in a significant amount of process emission that is a source of pollution.
Therefore, a need exists for a foam process which greatly reduces fugitive VOC emissions by reducing the concentration of the low permeability blowing agent (preferably a VOC blowing agent) employed, while still maintaining a same extrusion output rate as a conventional process using primarily a hydrocarbon or halocarbon blowing agent.